kmemcheck: remove duplicated #include

Remove duplicated #include in arch/x86/mm/kmemcheck/shadow.c.

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Acked-by: Pekka Enberg <penberg@cs.helsinki.fi>
Signed-off-by: Vegard Nossum <vegard.nossum@gmail.com>